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Impedance matching preamp
Impedance matching preamp
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Old 20th January 2019, 08:13 PM   #11
kodabmx is offline kodabmx  Canada
Affordable triode connected sweep tube amps.
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Impedance matching preamp
2019-01-06 14-06-26.jpg. 2019-01-06 15-12-08.jpg
Yup... 872mV.

The buffer is to spilt the input for outputs in parallel (computer, headphone amp, minidisc, cassette, EQ loop) and the gain stage is to drive my amps (about 6V for full power)

The gain stage is 300R. The buffer is about the same.

Relay volume control, DAC, Bluetooth etc all built in.

Last edited by kodabmx; 20th January 2019 at 08:15 PM.
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Old 20th January 2019, 08:35 PM   #12
jan.didden is offline jan.didden  Europe
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Quote:
Originally Posted by kodabmx View Post
Attachment 730471

Attachment 730472
The buffer is to spilt the input for outputs in parallel (computer, headphone amp, minidisc, cassette, EQ loop) and the gain stage is to drive my amps (about 6V for full power)
.
Ahh. Makes sense.

Jan
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Old 20th January 2019, 10:30 PM   #13
googlyone is offline googlyone  Australia
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Impedance matching preamp
Kodabmx,
That is an extraordinary box of wires you have there!!!

There is a fair bet that the 128mV reduction in signal level is just some the took a wrong turn in that wiring!
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Old 21st January 2019, 02:18 AM   #14
GoatGuy is offline GoatGuy  United States
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KodaBMX... hopefully you'll get a laugh...
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Old 21st January 2019, 04:00 AM   #15
H713 is offline H713  United States
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I don't have the guts to shove that much into a small chassis...

I built a 6L6 PP amp into a pretty small rackmount chassis thinking that "oh, it's a 2u. That should compensate for not being very deep". Never again. I'm still trying to herd my courage back together to take another crack at that thing.

That's about twice as dense... how did you get the noise floor down to a reasonable level?
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Old 21st January 2019, 07:28 AM   #16
Pucur is offline Pucur  Serbia
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Originally Posted by rayma View Post
There's no need for a transformer, take it out and make a direct connection. If your preamp cannot handle 470k, it's grossly defective.
Thank you all for help! I would like one more advice. I need gain of 2-3 times, is it the best way to do it with 2 parallel cathode followers or there is some better way?

I think that I'll try preamp with output transformers just to learn something and see what I can get. I have some idea, can I use resistors to adjust (lower) impedance if it is too big? See this: http://www.jensen-transformers.com/w...4/08/as010.pdf
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Old 21st January 2019, 10:44 AM   #17
DF96 is offline DF96  England
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A cathode follower has no gain. Paralleling them or cascading them will not add gain.

You need to take a step back, take a deep breath, and forget about transformers. There is no need for impedance matching in audio, in fact impedance mismatching is the requirement. Transformers are only used for two purposes:
1. when you really really need what only a transformer can do, and are therefore prepared to tolerate the significant downside of a transformer
2. when you want to be part of a fashion crowd and don't know or don't care what damage a transformer will do to your music signal
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Old 21st January 2019, 11:41 AM   #18
Zoran is offline Zoran  Serbia
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Design which includes, high step-down ratio transformer at the end. Should be reconsidered... For preamplifier, OR any smaller signal device, this means that before step down transformer we have amplification active circuit. With total Gain of stepdwn ratio + neded gain. That is no-need amplification, introducing distortion, larger values of Rload at the anode path, lower plate voltages, using Higher mju valves with higher internal resistance... etc etc.
...
I think that is not good aproach at all. Except when we just cant avoid - output step down transformer to link the tube and speaker...
...
I would think of conception with 1:1 transformer, input or output, and lower gain tubes. With buffer at the output IF needed. Because in that way You can correct the phase shift of 180deg from amplifinig stage, no squared transfer of any reactive component from P to S and opposite, just factor 1, and for my opp. maybe more important NO overamplification in the stage giving the specific sound signature.
.
That is just my perception on this topic. cheers.
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Old 21st January 2019, 01:14 PM   #19
GoatGuy is offline GoatGuy  United States
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Quote:
Originally Posted by Pucur View Post
Thank you all for help! I would like one more advice. I need gain of 2–3 times, is it the best way to do it with 2 parallel cathode followers or there is some better way?

I think that I'll try preamp with output transformers just to learn something and see what I can get. I have some idea, can I use resistors to adjust (lower) impedance if it is too big?
№ 1: How do you know you need 2× to 3× gain?
№ 2: Which source is too weak, and needs pre-amplification?
№ 3: Is it the only source which is weak?
№ 4: Are any sources "plenty loud"?
If (№ 2 or 3) are at their maximum and your amplifier isn't able to produce its full potential, well … then yes you need an amplifier. The answer to (№ 1) tho' is very important: how do you know? Oscilloscope? Or relatively crâhppy digital volt meter? (Don't worry, they're all "pretty crâhppy" at measuring millivolts of audio signal!!!)

It may be that you need 5× or 7×, for your weakest sources. Could be! In any case, if it is quantitatively measured and survives the sniff-test, well … then a preamp you need.

Ultimately, most people then have other decisions to make: is the preamp to replace the input switching capability of the system's amplifier? Or is it just to strengthen one weak source? Is the preamp desired to “color” the signal in a “tube-character” way? Or be absolutely neutral? Is the preamp to be visual statement and a talking point … or something invisible that quietly does its job?

Because then you are well armed. Transformers as DF96 says, are very rarely the solution. I will give them this: they sure are easy to implement.

For example, a pair of Hammond 1140-LN-C 4 to 1 ratio, or Jensen transformers, in a small aluminum box with RCA connectors for input and output. No volume control … just transformers-in-a-box. Never mind impedance-matching. It'll still set you back $250 or so. But can be built "in an evening". Nice little thing to build, as a proof-of-concept. And a valuable tool in your "bag of tricks"!

Just saying,
GoatGuy
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Last edited by GoatGuy; 21st January 2019 at 01:18 PM. Reason: typos
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Old 21st January 2019, 03:19 PM   #20
kodabmx is offline kodabmx  Canada
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Quote:
Originally Posted by GoatGuy View Post
KodaBMX... hopefully you'll get a laugh...
Haha!

1: A trimmer would have worked but I didn't have one.
2: It's mounted on a stand-off and switches out a 250R resistor from the power supply (otherwise the SMPS has to re-fire). The blue board above it is a latching relay circuit for switching COAX or Optical on the DAC. The orange board is a time delay driving the black relay. The power supply board for the DAC is under them.
3: That's the Bluetooth board.
4: Relay volume control.
5: It's the first main point for 280V B+.
6: It's the main point for 12V
7: Relay inputs, there are 4 balanced (that I use single ended, but switch grounds) and a 6 unbalanced board underneath it.
8: Of course! It's de-rated to 25% since it's in a metal box with no ventilation.

Since I took the photo, I've added an effects/tape loop handled by an octal relay. I had to raise the VU meter driver board and put the socket under it. Thankfully, there are no more additions planned. I ran out of 1" stand-offs.

Last edited by kodabmx; 21st January 2019 at 03:30 PM.
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